This invention relates to an improved electrophotographic copying machine and a method of controlling the machine.
An electrophotographic copying machine normally produces a copy through the following processes:
(a) Electrical charging process in which a photoconductive insulating surface (hereinafter referred to as "photosensitive element" or "photosensitive drum") is subjected to corona discharging in order to apply electric charges uniformly on the surface.
(b) Exposure process in which an original to be copied is exposed to light and the light reflected from the original is projected onto the photosensitive element through an optical system including a mirror and a lens. The electrical charges on the photosensitive element are thus converted to a pattern of electrical charges which is an electrostatic latent image corresponding to the original.
(c) Developing process in which a developer consisting of a toner or combination of a toner and a carrier is brought into contact with the area on the photosensitive element on which the pattern of electric charges lies, so that the pattern is converted to a visible image or a toner image.
(d) Transfer process in which the toner image is transferred onto a transfer sheet: for example, plain paper supplied in synchronism with the copying operation, by applying corona discharge to the transfer sheet from the back side thereof while the transfer sheet movingly overlaps with the area of the toner image on the photosensitive element.
(e) Fixing process in which the toner image on the transfer sheet separated from the photosensitive element is heated to melt and fix the toner image onto the transfer sheet.
In connection with the transfer process in the above item (d), using rollers as a sheet supply means is a common practice. Such sheet supply rollers operate in response to a copying operation starting signal--for example, depression of a copying button--and is always kept in contact with the uppermost sheet of the transfer sheets. Conventionally, these rollers are intermittently driven by their exclusive motor.
However, it is not desirable to provide such an exclusive motor for the sheet supply rollers in view of the fact that a small and inexpensive copying machine is desired, because the provision of such an exclusive motor may constitute a factor against the above desire. It is also a drawback that the provision of the exclusive motor causes a complexity of mechanisms involved in the transmission system of the copying machine because of the given worms and worm wheels disposed between the output shaft of the motor and the shaft of the sheet supply roller.
Synchronized control of the transfer sheet in exactly overlapping relation with the area of image on the photosensitive drum will now be described.
Heretofore, a pair of carrying rollers are provided in the passage of movement of a transfer sheet in the position nearer to the sheet supply station that the transfer electrode and are kept stationary until the front edge of the transfer sheet is pinched between the carrying rollers. Then, the carrying rollers are actuated by means of a synchronous signal obtained in association with rotation of the photosensitive drum, to transport the transfer sheet and cause overlapping thereof.
According to this system, the carrying rollers are actuated intermittently and operate to bring the transfer sheet into a temporary waiting condition. It is therefore difficult to control the carrying rollers so that the transfer sheet is transported in the same condition on every occasion of re-transportation thereof. This results in positional shift of the area of an image to be transferred onto the transfer sheet.
It is therefore intended in the present invention to provide an electrophotographic copying machine having no such disadvantages as mentioned above, and a method of controlling the machine.